09 January 2013

Researchers simulating humans living in a closed habitat while on deep space travel found that the human body balances its sodium levels in a cycle that is independent on the actual salt intake. They have published their findings in the journal Cell Metabolism.

Sodium is used by the body to regulate blood pressure and control blood volume. It also is used for osmotic equilibrium and pH. A cell is said to be in osomotic equilibrium when there is no water loss or gain while in the body. Sodium ions are used by the central nervous system for the proper function of its nerve cells.

Salt (Sodium chloride) is the prime source of sodium in the diet. It contains 40% sodium. The minimum physiological requirement for sodium is 500 milligrams per day while the DRI (Dietary Recommended Intake) is 2300 milligrams per day. Anything above 3400 milligrams may cause hypertension.

The body through the kidneys (as part of its renin-angiotensin system) regulate the sodium concentration of the body. As mentioned in the article (detailed below), the textbook explanation of sodium regulation is that when the body detects an uneven concentration of sodium in the fluids, the kidney will either retain or decrease water to maintain the optimal concentration. Thirst is a sign that the body needs more water to maintain this balance.

A 150 pound (70 kilogram) person would have 15 liters (~4 gallons) of water in his body. The sodium content maintained by the body in the water would be about 1.75 ounces (50 grams). Rhythmic Fluctuations of Sodium Levels in the Body

Maintaining the right sodium levels in the body is crucial for controlling blood pressure and ensuring proper muscle function. Conventional wisdom has suggested that constant sodium levels are achieved through the balance of sodium intake and urinary excretion, but a new study in humans published by Cell Press on January 9th in the journal Cell Metabolism reveals that sodium levels actually fluctuate rhythmically over the course of weeks, independent of salt intake. This one-of-a-kind study, which examined cosmonauts participating in space-flight simulation studies, challenges widely accepted assumptions that sodium levels are maintained within very narrow limits.

"The study highlights the importance of measuring salt excretion in urine over a longer time period to accurately estimate salt intake," says senior study author Jens Titze of Vanderbilt University School of Medicine. "This information is very important, given the emphasis on salt intake in terms of risk for cardiovascular disease and healthcare outcomes."

Past studies in humans have shown that when dietary salt intake increases, a steady state of sodium levels in the body is achieved through rapid urinary excretion. This process is under the control of a hormone called aldosterone, which causes sodium to be retained in the kidneys. However, most of these studies were short-term and did not examine fluctuations in sodium levels in response to constant salt intake.

Video: How Do We Use Sodium?

To address these limitations, Titze and his team took advantage of a unique opportunity to control salt intake and study salt balance over the course of nearly seven months in 12 male participants in the Mars105 and Mars520 studies. These men spent 105 and 520 days, respectively, in an enclosed habitat consisting of hermetically sealed interconnecting modules at a spaceship simulation facility in Moscow, where they lived and worked as if they were cosmonauts at the international space station. As expected, their aldosterone levels increased when they consumed less salt. But surprisingly, a decrease in salt intake led to a reduction in levels of the "stress hormone" cortisol.

When the researchers kept salt intake constant, sodium excretion and the levels of aldosterone and cortisol fluctuated together in weekly cycles. On the other hand, sodium levels in the body exhibited longer-term rhythmic changes that were independent of salt intake. Over this longer timescale, elevated sodium levels were associated with high aldosterone levels and low cortisol levels, suggesting that the two hormones work in opposite ways to control sodium storage and release.

"To the best of our knowledge, the long-term rhythm we observed in sodium levels in the body has not been previously reported, and it was not known that cortisol and aldosterone work in opposite directions to regulate sodium metabolism," Titze says.